In recent years, light-sensitive recording system, magnetic recording system and heat-sensitive recording system have been increasingly developed and put in practical use. In particular, the heat-sensitive recording system has advantages in that (1) the formation of color is performed in one step, and therefore, no development step is needed, (2) the construction and maintenance of an apparatus for use in the heat-sensitive recording system are easy, and (3) a heat-sensitive recording sheet is similar in the touch to that of ordinary paper and is inexpensive. Thus, the heat-sensitive recording system is now utilized in various fields such as an output section of a computer, a printer section of an electronic calculator, etc., a recording section of a medical measuring apparatus, a facsimile, an automatic ticket-vending machine, and a heat-sensitive copying apparatus.
Light-sensitive recording paper, magnetic recording paper and heat-sensitive recording paper for use in above systems are usually prepared by the same manner as used in the preparation of usual coated papers such as art paper and coat paper. In preparing such art and coat papers, a coating material prepared by dispersing an inorganic pigment (e.g., barium sulfate, titanium oxide, calcium carbonate, satin white, clay or kaolin) along with a binder, (e.g., casein, starch or synthetic latex) in water is coated on a base paper, and then, dried. In addition, mechanical treatments such as supercalendering are applied thereto. In general, such binders have also an ability to permit uniform dispersion of the above-described pigment in the coating material. In some cases, a dispersant is additionally used.
Similarly, light-sensitive recording papers, magnetic recording papers and heat-sensitive recording papers are prepared by dispersing uniformly a light-sensitive substance (e.g., diazonium compound), a magnetic substance (e.g., .gamma.-iron oxide) and a mixture of a leuco dye-type color former (e.g., a fluoran dye), and a color developer (e.g., a phenol compound), respectively, in water along with a binder to prepare the corresponding coating materials, and coating the coating materials on a base paper.
Binders for use in coating materials which are used in the preparation of such coated papers are desired to meet various requirements such as (1) they permit easy and good dispersion and adhesion of various additives such as pigment, dye and magnetic powder in a coating material, (2) they provide a uniform and strong coating film, (3) they have good water resistance, (4) they have good viscosity stability, (5) they have excellent workability, and (6) they are inexpensive. In addition, the binders for use in leuco dye-type heat-sensitive recording paper are required to exhibit good response to heat, to prevent formation of fog and deterioration of a thermal head, and to prevent the recording paper from sticking to the head. Further, it is desired for the binder used in magnetic recording paper to permit orientation of magnetic materials upon coating on a base paper and to have high bonding force whereby good abrasion resistance of the recording material can be attained with a small amount of the binder, resulting in an improved output level and running property.
Conventional binders include water-soluble substances such as polyvinyl alcohol, starch, and modified products thereof, casein, gelatin, soybean protein, carboxymethyl cellulose, gum arabic, methyl cellulose, hydroxyethyl cellulose, sodium alginate, polyacrylamide, poly(sodium acrylate), and an alkali-modified product of a styrene-maleic anhydride copolymer, and aqueous emulsions of a styrene-butadiene copolymer, a styrene-butadiene-acrylonitrile copolymer, an ethylene-vinyl acetate copolymer, a vinylidene chloride-acrylate copolymer, a vinylidene chloride-acrylonitrile copolymer, an acrylonitrile-acrylate copolymer, a polyurethane resin, a chlorinated polypropylene resin, a butadiene-acrylonitrile copolymer, and a vinyl acetate resin.
Water-soluble substances as described above (other than styrene-maleic anhydride copolymer alkali-modified products), however, have poor water resistance. They are, therefore, not necessarily satisfactory since various problems arise although they are usually used in combination with an agent to improve the water resistance thereof. On the other hand, styrene-maleic anhydride copolymer alkali-modified products and synthetic resin-based aqueous emulsions have poor bonding force, though they have good water resistance. Further, when the styrene-maleic anhydride copolymer alkali-modified products are used as a binder for the heat-sensitive recording paper, it suffers from the defect that it easily causes the deterioration of a thermal head. Also, the synthetic resin-based aqueous emulsions have disadvantages for the heat-sensitive recording paper in that they cause the formation of fog and because of their poor heat resistance the heat-sensitive recording paper sticks to a thermal head during recording, and disadvantages for the magnetic recording paper in that they do not have sufficient bonding force, providing poor abrasion resistance, low output level and poor running property. Thus, almost all conventional binders are not sufficiently satisfactory for practical use.